thyroid tests

This page is under review - 11-1-2021

Laboratory tests for thyroid disorders need to be carried out to:
1. Check if your thyroid is working normally
2. Find any fault
3. Determine if the problem is mild or serious
4. To find the correct dosage of any medication
5. Monitor progress     
However, no test is 100% accurate.

Standard Thyroid Tests are:
FT4 - Free Thyroxine - NZ Reference Range 12.0 - 22 pmol/L
FT3 - Free triiodothyronine - NZ Reference Range 3.1 - 6.8 pmol/L
RT3 - Reverse T3
TSH - Thyroid-stimulating hormone – NZ Reference Range 0.27 - 4.2 mIU/L
TPO - Thyroperoxidase
Tgab- Thyroglobulin antibodies

T4 - measures the amount of T4 in the blood, both free and attached to proteins.  If the reading is below 12 standard units, it suggests an underactive thyroid; if it is more than 22 standard units, this could mean an overactive thyroid, but other things may not be equal. A shortage of carrier protein will give a low result and an excess of protein will give a high result. Even so, the thyroid may be working correctly.

Increased protein and hence a high reading may be caused by many problems:
- Pregnancy, HRT (or contraceptive pill)
- Hepatitis
- Porphyria
- Cannabis
- Heredity
Reduced protein giving a low reading could be caused by:
- Steroids used in treatment
- Bodybuilding steroids
- Nephrosis
- Cirrhosis
- Heredity
To check if an abnormally high or low T4 level is due to the thyroid, there are a couple of choices - the TSH or the FT4 test.

TSH test - This is often used instead of, rather than as well as the T4 test. If the thyroid is not producing enough hormone for the body's requirements TSH comes into action to make the thyroid increase production. If the TSH result is above the critical level (2 or more in either international or standard units), the thyroid is underactive.

FT4 TEST - Some medical practitioners believe that this is the true measure of thyroid activity - other practitioners do not believe this test should be used alone, without taking into account clinical examination and symptoms.

T3, FT3 & RT3 tests - A high FT3 level occurs in hyperthyroidism or Grave’s Disease. With a healthy thyroid, increasing age causes a slow reduction in T3, unlike T4 and a number of illnesses have the same effect - low T3 syndrome. Fasting, starvation and anorexia nervosa all induce a low T3 level with a corresponding increase in reverse-T3.
TPO and Tgab are tests for thyroid antibodies produced by the system itself, and above normal levels indicate autoimmune thyroiditis (Hashimoto's disease).                                    

From a patient’s point of view, these are simple - just an ordinary blood test. For the laboratory, they are complex. Antibodies in the blood indicate susceptibility to autoimmune thyroid problems.
1. Graves' Disease.  The main responsible antibody is TRAb - thyroid receptor antibody. If there is a high level of this during pregnancy, it is a warning to take action to protect the unborn baby.
2. Hasimoto's Disease.  Several antibodies are involved, including anti-Tg (antithyroglobulin) and anti-M (anti-microsomal). The majority of Hashimoto suffers carry these antibodies, but so do 1 in 5 people who have never had a thyroid problem. This last group may, however, be more susceptible to developing an autoimmune disorder if the thyroid is put under stress, by, for instance, lithium medication, an infection or a faulty diet.

24 HOUR URINE THYROID HORMONE TESTING - The 24-hour urine thyroid test serves as a valuable tool for detecting thyroid dysfunction that can go undetected through the usual standard blood tests. It is important to use this test in conjunction with other indicators of thyroid function, such as body temperature, symptoms and standard blood thyroid tests.

RADIOACTIVE UPTAKE (RAIU) TEST - This test how effectively the thyroid cells are latching on to the iodine in the circulation, which is a necessary ingredient of thyroid hormones. The test starts with a scan of your basic level of radioactivity, with a sort of Geiger counter. They you are given a measured dose of a mildly radioactive form of iodine in a capsule or as a liquid. The thyroid area is then scanned again at various intervals up to 24 hours to see how much of the iodine has been taken up. For a quicker test the follow-up scan can be done three to four hours after the start, but in this case you must do without food during the whole time.
The results are useful in diagnosis and also in assessing the dosage necessary if radio-iodine treatment is in view.

High uptake will result from :-

- Graves disease and other overactivity
- Iodine deficiency
- Having stopped antithyroid drugs
- A diet full of soy
- Kidney disease
Low uptake will result from :-

- An underactive thyroid
- Medication containing iodine
- Diet - iodine-enriched foods or vitamins products
- Taking thyroxine - you must stop one month before the test
- Previous radioiodine treatment or thyroid operation
- Old age
- Having just exercised very energetically

The radioactive iodine used for the RAIU test has nothing like the strength of that used in treatment. Its radioactivity only lasts for 3-4 days. Another radioactive material, technetium, is sometimes used instead of iodine; it is given by injection. Whichever material is used, the test is unsuitable for young children or anyone who might be pregnant, even at this low level of radiation.

SCINTIGRAM This technique uses a special camera to produce a brightly multicoloured picture showing where iodine is taken up by thyroid tissue, and how vigorously. Like the RAIU test, it depends on having a measured amount of the weak radioiodine,123I, or technetium 99m first. In a few centres fluorescent scanning is available: this measures ordinary, non-radioactive iodine through something like an X-ray, and almost no radiation is involved.
Also called photoscan, scintigraph, scintiscan.

This test is useful:

- To show the size and shape of the gland.
- To check for thyroid tissue behind the breastbone.
- To find out whether a lump in the tongue or neck is thyroid tissue that has gone off course during development.
- Most importantly, to provide information on a particular knob or lump of tissue in the thyroid.
- a 'hot' nodule (showing as red) is overactive, taking in a lot of iodine;
- a 'warm' nodule (showing as orangey-yellow) is normally active;
- a 'cold' nodule (showing as greenish) is not taking up iodine and may be a cyst or a tumour. This calls for further investigation, to exclude cancer.

X-RAY - An ordinary X-ray gives a shadowy idea of the size and position of the thyroid. In particular, a chest X-ray may reveal a shadow, behind the breastbone, which could be an extension of thyroid tissue. Ultrasound, a CY scan or a scintigram will be needed for more precise information. 

BARIUM SWALLOW - This is an X-ray taken while you are swallowing a barium drink that shows up on X-ray. It reveals any pressure on your gullet.

CT (COMPUTERISED TOMOGRAPHY ) SCAN - This is an X-ray that presents what looks like pictures of slices through the next or other area.

ULTRASOUND - This is a simple painless method of obtaining a picture of an internal organ, including the thyroid. It produces an on-going picture by processing the echo of a high-frequency sound projected on to the organ. Apart from distinguishing a cyst from solid tissue, the ultrasound provides an ongoing image of the organs and structures in your neck. This is invaluable for guiding the needle when a biopsy of a particular part of the thyroid is required.    

FINE NEEDLE ASPIRATION - This is a method of doing a biopsy to obtain a sample of tissue to examine under the microscope and identify precisely. The great value of FNA is for distinguishing between a commonplace nodule of normal thyroid tissue, a harmless cyst or benign growth and a cancer. This knowledge is a signpost to the best form of treatment.                                              

METABOLIC RATE - Although the main work of the thyroid is controlling the rate at which the bodily processes use up the available nourishment, the metabolic rate is seldom tested. A raised basic metabolic rate goes with over-activity of the thyroid, and accounts for the person who eats enormously and stays thin. The opposite occurs with an under-active thyroid.

ELECTROCARDIOGRAPH - This electrical tracing of the heart's activity is the standard method of assessing how well the heart is working. It shows characteristic changes in overactive and underactive thyroid.

OPTHALMIC CURVE METER - This apparatus measures the degree of protrusion of the eye or eyes. 
Clinical Chemistry, Toxicology, Seriology - reprinted from health Reference - 2001